1. Field of the Invention
This invention is in the field of microcontrollers and methods of operation therefore and, more particularly, is a microcontroller having special mode enable detection circuitry and a method of operation therefore.
2. Description of the Related Art
Special modes can be used with microcontroller devices for a variety of reasons, the most common of which is to test the device in a non-user environment (i.e., when the device is not in normal operation). Due to the common use of special modes as a means for testing a device, some simply refer to the special operating mode as a test mode. It is known to those skilled in the art that functions other than device testing can be done in the special mode. Such other functions might include, but are not necessarily limited to, the built-in self-test (BIST) function, and device initialization in normal user (i.e., non-test) mode.
In order to enter a special operating mode, those in the past sometimes simply added an independent pin to the microcontroller that had the exclusive function of indicating that the device is to enter test or special operating mode. After applying power to the device, the dedicated special mode pin is set to a level that indicates to the device that it should stop normal execution, and begin operating in special mode. This approach has the obvious drawback that it wastes space (i.e., the dedicated pin for detecting special mode), and therefore, if space constraints are a concern for a device, this approach is not desirable. In an alternative prior art approach, there is no dedicated test mode detection pin for the microcontroller, but rather a shared pin that functions both as an Input/Output (I/O) pin and as a pin for detecting the test mode. This shared pin approach works as follows. When the test mode is to be entered, a voltage above V.sub.DD is applied to the shared pin to indicate that a test mode is to be initiated; however, such a technique requires that the microcontroller has some means for stopping normal device operation before testing--in other words a reset signal or the like is required. The situation at issue in the instant invention does not require a reset signal to interrupt normal microcontroller operation. Thus, this second prior art approach would not work in the situation involved with the instant invention.
In contrast to the prior art, the invention here arose from the necessity to enter a test mode on a microcontroller device that has a low pin count, and no means of keeping the device from free-running execution after applying power to the device. Here, the limitation of a "low pin count" means that the chip designer does not want to waste a pin for the exclusive function of identifying the initiation of the test mode--this could be the case in a microcontroller having very few pins (such as 8) or many (such as 64). The point is that the specific number of pins is not relevant. What is relevant is the fact that the designer doesn't want to waste space with using a dedicated pin for detecting the test mode. Further note that the situation here that necessitated the instant invention included the limitation that there was no means for the microcontroller to keep itself from free-running execution after applying power to the microcontroller device. In other words, the situation here involved a microcontroller that would not have a reset signal, either from an external reset signal input to the device via a reset pin or via some type of on chip software-generated reset signal well known to those skilled in the art. Since no reset signal was available for the microcontroller here involved, once V.sub.DD was applied to the device, there was no way to stop it, and enter a test mode for the device. Thus, for the purpose of discussion here, the term "free-running microcontroller" means a microcontroller without an externally provided reset signal, and without any type of on chip software or hardware induced reset signal for the microcontroller, or at least if such a software or hardware induced reset signal could be established, it has not been so established. In other words, a free-running microcontroller (which is the area of concern for the instant invention) has no reset signal to stop it from running once V.sub.DD has been applied to the microcontroller.
Therefore, there existed a need to provide a free-running microcontroller that can enter the test or special operations mode without the benefit of a dedicated pin for such purposes, and a method of operation therefore.